Generally, hot restartability of a small internal combustion engine provided with diaphragm carburetor is not good for several reasons mentioned below:
(1) A metering chamber is heated by heat of the engine, atmosphere heat, radiant heat of sunshine and the like after the engine has been suspended. This occurs conspicuously particularly after operation with high load under a burning sun in a summer season. If the metering chamber is heated as described above, fuel having a low boiling point interiorly stored is changed into vapor and flows from a fuel passage to an air inlet and a venturi portion. At that time, liquid fuel having a high boiling point also flows out together, which stays as vapor liquid in the air inlet and venturi and flows into a crank case depending on the attitude of the engine.
(2) Particularly, in 15 to 20 minutes or so after the operation of the engine has been suspended, the fuel in the metering chamber completely flows into the air inlet, and the interior is filled with fuel vapor.
(3) When a recoil starter is pulled to restart the engine, fuel remaining in the air inlet and venturi portion is taken into the engine all at once and supplied in the form a super-rich mixture. Therefore, the engine will not start.
Particularly, at the time of restarting the engine within a period of 15 to 20 minutes after the engine has been stopped, the engine is still in a hot state requiring no rich-mixture, and therefore, when the super-rich mixture is supplied, the engine is more difficulty to start.
(4) In such a state as described above, roping, that is, pulling the starting cable of a recoil starter, is carried out for several times to discharge the mixture, and the initial explosion could be effected only when the fuel mixture in the interior of the cylinder is in the range of combustion.
(5) When a throttle valve is opened and roping is effected at a start position, a mixture may be exhausted with less roping to effect the initial explosion. However, since the throttle valve is opened, venturi pressure is sufficiently low that it is difficult to clear the vapor in the metering chamber, and even if the initial explosion is effected, the engine is not possible to continue running but stops soon. Even if roping is effected over and over again thereafter, the engine will not start.
(6) In the case where the throttle valve is in a less open position as in idling, roping has to be done over and over again to exhaust the rich mixture from the venturi passage. A spark plug may become covered with the mixture depending on the displacement of the engine and the position of the ignition plug, and this also makes restart more difficult.
(7) The outflow of fuel in the air inlet and venturi portion of the metering chamber after the operation of the engine has been suspended makes it difficult to provide hot restart even if the throttle valve portion is opened and even in the idling position.
(8) When the choke valve is used in the state wherein the engine is hot, fuel remaining in the air inlet is supplied in its richer state to the engine and therefore the engine is again difficult to restart.
As one means for solving these problems noted above, the present applicants have previously proposed a restarting fuel supply device provided with a diaphragm carburetor for the internal combustion engine in which a jet is provided in the midst of a hose connecting an upper opening of a fuel tank and an opening of an intake pipe adjacent to an air inlet of the engine, an air intake is provided at a downstream of the jet, the air intake and the jet being normally closed, and at the restart under high temperature, the air intake and the jet are opened, and at the same time, air is introduced from the outside to the bottom portion internally of the fuel tank through a check valve, said air being introduced as bubbles from a porous member to the inside of the fuel.
In the above-described restarting fuel supply device provided with a diaphragm carburetor for the internal combustion engine, even if the engine is in the high temperature state experienced after the operation has been suspended or the like, the opening and closing valve of the restarting fuel supply mechanism may be opened to thereby supply fuel gas (vapor) at the upper portion of the fuel tank together with air taken from the air inlet to the intake pipe at the downstream of the carburetor. However, in the aforesaid restart state, since the intake passage is low in intake pressure, it is not possible to draw the fuel vapor satisfactorily, and, in addition, a measure for preventing fuel collection in the metering chamber, after the engine has stopped, from being heated by the heat of the engine or environmental heat and flowing into the intake passage has not been taken. Thus, super-rich fuel is supplied to the intake passage at the time of restart, and the hot restartability of the engine is not always good.